Multi-bit ROM cell, for storing one of n&gt;4 possible states and having bi-directional read, an array of such cells, and a method for making the array

ABSTRACT

A array of multi-bit Read Only Memory (ROM) cells is in a semiconductor substrate of a first conductivity type with a first concentration. Each ROM cell has a first and second regions of a second conductivity type spaced apart from one another in the substrate. A channel is between the first and second regions. The channel has three portions, a first portion, a second portion and a third portion. A gate is spaced apart and is insulated from at least the second portion of the channel. Each ROM cell has one of a plurality of N possible states, where N is greater than 2. The state of each ROM cell is determined by the existence or absence of extensions or halos that are formed in the first portion of the channel and adjacent to the first region and/or in the third portion of the channel adjacent to the second region. These extensions and halos are formed at the same time that extensions or halos are formed in MOS transistors in other parts of the integrated circuit device, thereby reducing cost. The array of ROM cells are arranged in a plurality of rows and columns, with ROM cells in the same row having their gates connected together. ROM cells in the same column have the first regions connected in a common first column, and second regions connected in common second column. Finally, ROM cells in adjacent columns to one side share a common first column, and cells in adjacent columns to another side share a common second column.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 10/642,079 filed on Aug. 14, 2003 now U.S. Pat. No. 6,927,93issued on Aug. 9, 2005, the subject matter of which is incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to a multi-bit ROM cell for storing one ofn (n>4) possible states, an array of such ROM cells and a method formaking such an array. Further, the present invention relates to such amulti-bit ROM cell array in which each cell is read bi-directionally.

BACKGROUND OF THE INVENTION

A Read-Only Memory (ROM) cell is well known in the art. Typically, a ROMcell comprises a single MOS transistor having a first region, and asecond region separated from one another by a channel. A gate ispositioned over the channel and is insulated therefrom. A voltage isapplied to the gate and the voltage controls the conduction of thechannel. A single bit ROM cell means that the V_(TH) or the voltage ofthe threshold by which the transistor turns on has been adjusted by animplantation step. When an appropriate voltage is applied to the gate,the source, and the drain, either the ROM cell is turned on or is turnedoff. Thus, the ROM cell is capable of storing a single bit.

A ROM cell capable storing multi-bits is also well known in the art. Theadvantage of a multi-bit ROM cell is that the density of the memorystorage can be increased. Referring to FIG. 1, there is shown a typicalprocess for manufacturing a ROM cell for storing one of a plurality ofbits. The ROM cell 10 has a source 12, a drain 14 spaced apart from thesource 12 and a channel 16 therebetween. The source 12 and drain 14 arein a substrate 20. Typically, the substrate 20 is of a p-typeconductivity. Thus, the source 12 and drain 14 are of n-type. Of course,the substrate 20 can also be a well within the substrate 20. A gate 22is spaced apart and insulated from the channel 16 by an insulation layer24. If the ROM cell 10 is to store, e.g. two bits or four possiblestates, the ROM cell 10 would have to undergo potentially as many asthree masking steps for implantation. One of the possible states for theROM cell 10 is in which the V_(TH) (designated as V_(T1)) is thehighest. In that event, no additional implant of N type material is madeinto the channel region 16 thereby affecting the V_(TH). The next higherlevel of V_(TH) would be an implant of donor (n−) species into thechannel region 16. A third and fourth state would be where yet evenhigher dosages of donor (n−) species are implanted into the channel,lowering V_(TH). Thus, if the ROM cell 10 were to store one of apossible of four states representing two bits, potentially, as many asthree additional mask steps would be required to implant the channelregion 16 to change the V_(TH) thereof. An array of multi-bit ROM cellsis also well known in the art. However, similar to the foregoingdescription with regard to the manufacturing of a multi-bit ROM cell,the array is made with potentially as many as M−1 implants, with M asthe total number of possible states.

An MOS transistor is also well known in the art. Typically, an NMOStransistor 30, such as the one shown in FIG. 2A, comprises a sourceregion 32, a drain region 34 and a substrate 20. Again, the substratetypically is of P type conductivity and the source 32 and drain 34, areof N type. Again, the source 32 and drain 34 can be in a well, with thewell in the substrate 20. Further, the conductivity of the source 32,drain 34 and of the substrate (or well) can be reversed, and thetransistor 30 would be PMOS type. A channel 36 is between the source 32and drain 34. As the scale of integration increases i.e., as the size ofthe MOS transistor 30 decreases, typically the channel region 36 willhave three portions: each labeled as 1, 2 and 3 in FIG. 2A. A gate 22 isspaced apart from at least the second portion of the channel 36 by aninsulation layer 24. Because of the scale of integration, LDD (lightlydoped drain) structures 38 and 40 are formed in portions 1 and 3, withportion 1 located adjacent to and connected with the source region 32and portion 3 located adjacent to and connected to the drain region 34.The second portion is between the first and third portions. The LDD likestructures in portions 1 and 3, shown in FIG. 2A, are of the same typeof conductivity as the source and drain 32 and 34, respectively. Thus,in the event the substrate 20 is of P type and the source and drain 32and 34 are of N type, the LDD like structures (also known as“extensions”) in portions 1 and 3 are also N type. The function of theextensions is to decrease the resistance between the source 32 and thedrain 34, which increases the turn on current. Thus, a removal of eitherone or both of the extensions 38 and 40 in FIG. 2A would decrease thecurrent flow between the source and drain.

In addition, because of the increased scale of integration, halo regions42 and 44 have also been implanted into portions 1 and 3. A halo portion42 or 44 is an increase in conductivity of the same type as thesubstrate 20. Therefore, again, if the substrate 20 is of the p-type,and the source and drain 32 and 34 are of n-type, with the extensions 38and 40 also of n-type, the halo regions 42 and 44 are of p-type, butwith a concentration greater than the substrate 20. The halo regions 42and 44 prevent punch through. The effect of adding halo regions 42 and44 is to increase the V_(TH), which decreases the turn off current.Thus, removal of the halo regions 42 and 44 would reduce the V_(TH)thereby increasing current flow between the source drain 32 and 34respectively. This is shown in FIG. 2B. One can choose to include eitherthe halo regions 42 and 44 or the extensions 38 and 40, or both byselecting the biases to emphasize one effect versus another effect. Ifstandard CMOS masks are not used, however, then only one effect, i.e.either halo regions 42 and 44 or extensions 38 and 40 is chosen.

As can be appreciated, the formation of each of the extensions 38 and 40and of the halo regions 42 and 44 requires an additional masking step.

Accordingly, it is one object of the present invention to make an arrayof multi-bit ROM cells in which the operations of implant and masking isreduced compared to the method of the prior art.

SUMMARY OF THE INVENTION

A multi-bit Read Only Memory (ROM) cell comprises a semiconductorsubstrate of a first conductivity type with a first concentration. TheROM cell has a first region of a second conductivity type in thesubstrate and a second region of the second conductivity type in thesubstrate, spaced apart from the first region. A channel is between thefirst region and the second region with the channel having threeportions: a first portion, adjacent to the first region, a third portionadjacent to the second region, and a second portion between the firstportion and the third portion. A gate is spaced apart and insulated fromat least the second portion of the channel. The ROM cell stores one of aplurality of n (n>4) possible states, and is characterized by having oneof a plurality of threshold voltages in the second portion. Further, foreach threshold voltage the ROM cell has: (1) a first extension region inthe first portion of the channel adjacent to the first region, with thefirst extension region being of a conductivity type or a concentrationdifferent from the first conductivity type and the first concentration,and the third portion of the channel adjacent to the second region beingthe first conductivity type having the first concentration; or (2) asecond extension region in the third portion of the channel adjacent tothe second region, with the second extension region being of aconductivity type or a concentration different from the firstconductivity type and the first concentration, and the first portion ofthe channel adjacent to the first region being the first conductivitytype having the first concentration; or (3) the first extension regionin the first portion of the channel adjacent to the first region, withthe first extension region being of a conductivity type or aconcentration different from the first conductivity type and the firstconcentration, and the second extension region in the third portion ofthe channel adjacent to the second region with the second extensionregion being of a conductivity type or a concentration different fromthe first conductivity type and the first concentration; or (4) thefirst portion of the channel adjacent to the first region being thefirst conductivity type having the first concentration, and the thirdportion of the channel adjacent to the second region being the firstconductivity type having the first concentration.

The present invention also relates to an array of the foregoingdescribed multi-bit ROM cells.

The present invention also relates to an array of multi-bit ROM cellswherein the semiconductor substrate also has a MOS transistor with theMOS transistor formed during a masking operation. The one state of eachROM cell is made by a masking step which is also used to make the MOStransistor.

Finally, the present invention relates to a method of making such anarray of multi-bit ROM cells. The method comprises implanting thesubstrate to form a plurality of spaced apart first regions of a secondconductivity type, parallel to one another, in the column direction, inthe substrate. Each first region is the common column line betweenadjacent columns of ROM cells. The array is selectively masked, topermit implanting a certain select of the ROM cells to be implanted toone of four possible states, depending upon whether the first portion orthe third portion of the channel, if any, is implanted. Thereafter, themasked array is implanted to form the first, second, third or fourthstate of the ROM cells. The array is also masked to permit implanting afifth select of the ROM cells wherein for each ROM cell, implantingwould occur in its associated second portion of the channel for settingthe threshold voltage of the cell to one of a plurality of possiblevoltages. The array is then implanted to set the threshold voltages forthe fifth select of said ROM cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the method of the prior art tomake a multi-bit ROM cell.

FIGS. 2A and 2B are schematic diagrams showing a method of making an MOStransistor of the prior art.

FIGS. 3A–3D are schematic diagrams of one example of an improved ROMcell having four possible states.

FIGS. 4A–4D are schematic diagrams of another example of an improved ROMcell having four possible states.

FIGS. 5A–5D are schematic diagrams showing the operation of a readmethod to detect the state of a ROM cell of the type shown in FIGS.3A–3D.

FIG. 6 is a circuit diagram of an array of ROM cells with appropriateswitches and sensing circuits to read a select ROM cell.

FIGS. 7A–7L are cross-sectional, perspective diagrams showing a processof making an ROM array with each ROM cell having one of a plurality ofpossible states.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 3, there is shown one example of an improved multi-bitROM cell 50 in one of a possible of four states. The cell 50 isconstructed in a semiconductor substrate 20 such as single crystallinesilicon of the p-conductivity type, although it would be appreciated bythose skilled in the art that n-conductivity type material can also beused. Further, as used herein, the term “substrate” can also includewells that are in substrates. The substrate 20 has a first conductivitytype, such as p-type, having a first concentration level. The cell 50comprises a first region 32 and a second region 34 spaced apart from oneanother and each being of a second conductivity type, such as n+material, opposite the first conductivity type of the substrate 20.Between the first region 32 and the second region 34 is a channel 36having three portions. A first portion is immediately adjacent to thefirst region 32. A third portion of the channel 36 is immediatelyadjacent to the second region 34, with the second portion between thefirst portion and the third portion. A gate 22 is spaced apart andinsulated from the channel 36 by an insulation layer 24 and overlies atleast the second portion of the channel 36.

The ROM cell 50 has a certain threshold voltage in the substrate 20 inthe second portion of the channel. For each threshold voltage, the ROMcell 50 can have one of four possible states. In the first possiblestate, shown in FIG. 3A, the first portion and the third portion of thechannel 36 each has the same conductivity type and concentration as theconductivity type and concentration of the substrate 20. A second stateis shown in FIG. 3B. In the second possible state, an extension 40, of asecond conductivity type, is in the third portion and is connected toand is immediately adjacent to the second region 34, which also is ofthe second conductivity type. Typically, the extension 40 has a lighterconcentration of the second conductivity type than the second region 34.However, this limitation is not necessary, so long as the extension 40with the second conductivity type is present thereby changing the Vth orthe conductivity of the ROM cell 50 from that of the first state shownin FIG. 3A. The first portion continues to have the first conductivitytype with the first concentration, the same as the substrate 20. A thirdpossible state shown in FIG. 3C. In this state, an extension 38 is inthe first portion of the channel 36 and is immediately adjacent to andconnected to the first region 32. The extension 38 is of the secondconductivity type, same as the first region 32. The third portion of thechannel 36 has the same conductivity type and concentration as thesubstrate 20. A fourth and final state is shown in FIG. 3D. In thisstate, a first extension 38 of the same conductivity type as the firstregion 32 is in the first portion of the channel 36 and is immediatelyadjacent to and connected to the first region 32. A second extension 40also of the second conductivity type is immediately adjacent to andconnected to the second region 34 and is in the third portion. Thus, fora plurality of different threshold voltages in the substrate 20 in thesecond portion of the channel, there would be n possible states withn>4.

Referring to FIG. 4, there is shown another embodiment of a multi-bitROM cell 150 for storing one of a plurality of states. The ROM cell 150is similar to the ROM cell 50 shown and described in FIGS. 3A–3D. TheROM cell 150 comprises a first and second regions 32 and 34 spaced apartfrom one another of a second conductivity type in a semiconductorsubstrate 20 of a first conductivity type having a first concentration.A channel 36 is between the first and second regions 32 and 34. Thechannel has three portions with a first portion adjacent to the firstregion, a third portion adjacent to the second region, and a secondportion between the first and third portions. A gate 22 is spaced apartand is insulated from at least the second portion of the channel 36 bythe insulation material 24. The ROM cell 150 has a certain thresholdvoltage in the substrate 20 in the second portion of the channel. Foreach threshold voltage, the ROM cell 150 can have one of four possiblestates described as follows:

In the first possible state, the first portion and the third portion ofthe channel 36 are of the first conductivity and first concentration,the same as the substrate 20, and is of the same state shown anddescribed in FIG. 3A.

In the second possible state, a halo 42 is implanted and is formed inthe first portion of the channel 36 and is adjacent to the first region32. The halo 42 is of the first conductivity type as the substrate 20,but has a higher concentration than the substrate 20. The third portionof the channel 36 remains of the first conductivity type having a firstconcentration the same as the substrate 20.

In the third possible state, a second halo 44 is formed in the thirdportion of the channel 36. The halo 44 is of the first conductivity typebut has greater concentration than the concentration of the substrate20. The first portion of the channel 36 remains at the firstconductivity type with the same concentration as the substrate 20.

Finally, in the fourth possible state, halos 42 and 44 are formed in thefirst and third portions of the channel 36 with each of the halos 42 and44 being of the first conductivity type with a concentration greaterthan the concentration of the semiconductor substrate 20.

Thus, for a plurality of different threshold voltages in the substrate20 in the second portion of the channel, there would be n possiblestates with n>4.

Referring to FIG. 5, there is shown a series of schematic diagramsshowing how the ROM cell 50 or 150 can be read to determine its state.For the purposes of illustrating the read operation, it is assumed thatthe ROM cell 50 is of the type shown as described in FIGS. 3A–3D, i.e.extensions 38 and 40 are selectively implanted, depending upon the stateof the ROM cell 50, with the threshold voltage in the second portion ofthe channel at a certain level. Initially, if the substrate 20 is of theP conductivity type, and the threshold voltage in the second portion ofthe channel at a certain level, a positive voltage, such as 3.3 volts,needs to be applied to the gate 22. In addition, ground or V_(SS)<V_(DD)is applied to the first region 32 and V_(DD) or +3.3 volts is applied tothe second region 34. The application of a positive voltage to thesecond region 34 causes a depletion region 48 to be formed around thesecond region 34. The limits of the depletion region 48 is shown as adotted line 47 in FIGS. 5A–5D. If the ROM cell 50 were in the firststate, i.e., no extension regions were formed in either the firstportion or the third portion of the channel 36, then the resistance ofthe channel 36 is determined by the distance from the edge of the firstregion 32 to the limit 47 of the depletion region 48 formed about thesecond region 34, in series with the threshold voltage of the secondportion of the channel. This total resistance determines the V_(TH).However, as can be seen in FIG. 5C, even if the ROM cell 50 were in thethird state where a second extension 40 were formed (by implantation orother method) in the third portion of the channel adjacent to the secondregion 34, the depletion region 48 would overcome the second extension40. Thus, the distance between the first region 32 and the edge 47 ofthe depletion region 48 would be the same for the case where the ROMcell 50 were programmed to a state shown in FIG. 5A or to a state shownin FIG. 5C. both of these states would exhibit the same V_(TH) (assumingthe same threshold voltage in the second portion of the channel) andwould have substantially the same current flow under the conditions ofV_(DD) applied to second region 34, V_(SS) applied to first region 32,and a positive voltage such as V_(DD) being applied to the gate 22.

For the other two possible states (shown in FIGS. 5B and 5D), however,i.e., where the first extension 38 is formed in the first portion of thechannel 36 and is adjacent to the first region 32, the distance betweenthe edge of the first extension 38, closest to the second region 34 andto the outer edge 47 of the depletion region 48, is substantiallyreduced. Under this condition, the V_(TH) is less than V_(TH) of thestates shown in FIGS. 5A and 5C (again assuming the threshold voltagefor the second portion of the channel is the same). Thus, under thecondition of the same voltage applied to the regions 32, 34 and gate 22,as for the first case above, the current flow measured would be higherthan the two states shown in FIG. 5A or 5C.

Therefore, when V_(DD) is applied to second region 34 and to the gate 22and V_(SS) applied to first region 32, two possible current flows may bedetect for either the states shown in FIGS. 5A and 5C or for the stateof the ROM cell 50 shown in either FIG. 5B or 5D. Based upon thiscurrent flow detected, states shown in FIGS. 5A and 5C aredifferentiated from the states shown in FIGS. 5B and 5D.

Assume for the moment that the current flow is low, indicating that theROM cell 50 is in either of the states shown in FIG. 5A or 5C comparedto the states shown in FIG. 5B or 5D, the read method continues todifferentiate between states shown in FIG. 5A and FIG. 5C, by reversingthe voltages applied to the first and second regions 32 and 34. Thevoltage of V_(DD) would then be applied to the first region 32 and tothe gate 22 and the voltage of V_(SS) would be applied to the secondregion 34. A depletion region would be formed about the first region 32.Since for the case of the ROM cell 50 being in the state shown in FIG.5C, the V_(TH) is less than the V_(TH) of the state shown in FIG. 5A,the ROM cell 50 being in the state shown in FIG. 5C would generate ahigher current than the ROM cell 50 being in the state shown in FIG. 5A.The current flow measured with the application of these voltages wouldthen determine whether the ROM cell 50 is in the state determined byFIG. 5A or 5C.

As can be seen from the foregoing, with the ROM cell 50 or 150 and theformation of either the extension 38 or 40 or the halo 42 or 44, theextension or halo can be formed at the same time as the formation of theextension or halo in a conventional MOS transistor, such as shown anddescribed in FIGS. 2A and 2B. Therefore, in any integrated circuitdevice having a ROM cell, with MOS transistors (such as those used adecoding circuit or sensing circuit or the like) where the MOStransistors require the formation of extensions or halos, the formationof the state of a ROM cell 50 or 150, can be made at the same time asthe masking operation which is used to form the halo or the extensionsof a MOS transistor. This would reduce the cost in the formation of theROM cell 50 or 150. Further, by changing the threshold voltage of thesecond portion of the channel, by, e.g. implanting N type material toincrease the Vth in the second portion of the channel, the number ofstates that can be stored in a ROM cell 50 or 150 can be one of nstates, where n is greater than 4.

To differentiate the states associated with one threshold voltage forthe second portion of the channel, from states associated with anotherthreshold voltage for the second portion of the channel, assume thatthere are two possible threshold voltages for the second portion of thechannel: 1.5 volts, and 2.0 volts. Thus, there are a possible of 8 totalstates of storage. In the first method, 2.0 volts is applied to the gate22. If the ROM 50 or 150 has a threshold voltage in the second portionof the channel at 2.0 volts, then irrespective of the voltages appliedto source 32 and drain 34, no current flow (or insignificant currentflow) would occur between the source 32 and drain 34 (or vice versa).Then applying 3.3 volts to the gate 22 would cause current flow betweenthe source 32 and drain 34 and reversing the voltages applied woulddetermine one of the possible 4 states. If the ROM 50 or 150 has athreshold voltage in the second portion of the channel at 1.5 volts,then applying Vdd and Vss to source 32 and drain 34 and 2.0 volts to thegate 22, would cause a small amount of current to flow. However,applying the same Vdd and Vss to source and drain 34 and 3.3 volts togate 22 would cause more current to flow. Thus, the four states of theROM 50 or 150 with the threshold voltage of the second portion of thechannel at one level can be distinguished from the four states of theROM 50 or 150 with the threshold voltage of the second portion of thechannel at another level, based upon the amount of current flow.

Referring to FIG. 6 there is shown a schematic circuit diagram of a ROMdevice 70 having an array 60 of ROM cells 50 or 150. The array 60 of ROMcells are arranged in a plurality of rows and columns. A plurality ofrows 90, 92, 94 are attached to the gate of the ROM cells in each of therespective rows. Thus, the gates of all the ROM cells in the same roware electrically connected together. A plurality of column lines 62, 64,66 and 68 are connected to the first regions 32 of all the ROM cellsthat are arranged in the same column. The column line 62, 64, 66 and 68also connect all the second regions 34 of the ROM cells that arearranged in the same column. As can be seen from FIG. 6, each column ofROM cells that are adjacent to one another share a common column linewhich is connected to the second regions 34. Thus, the column line 64 isconnected to the second regions of the ROM cells located in the columnbetween the column lines 62 and 64 and to the second regions 34 of theROM cells located in the column between the column lines 64 and 66.Further, the column line 66 is connected to the first regions 32 of theROM cells located in the column between the column lines 64 and 66 andthe column line 66 connects all of the first regions 32 of the ROM cellslocated in the column between the column lines 66 and 68. As can beappreciated, the terms first regions 32 and the second regions 34 may beinterchanged. Further, as can be seen from FIG. 6, the array 60comprises a plurality of ROM cells with each ROM cell being programmedto one of a plurality of different states. Thus, for example, as shownin FIG. 6, the ROM cell whose gate is connected to row line 90 and whosefirst and second regions are connected to column lines 64 and 66 isindicated as having an extension region connected and adjacent to thecolumn line 64. (As used herein, including the claims, the term“extension region” means an extension 38 or 40 or a halo 42 or 44).Similarly, the ROM cell whose gate is connected to row line 92 and beingconnected to column lines 64 and 66, has an extension region which isconnected to the column line 66. Finally, the ROM cell whose gate isconnected to row line 92, but whose first and second regions areconnected to column lines 66 and 68, has extension regions connected toboth column lines 66 and 68. As previously discussed, these threeexamples of ROM cells all “store” states that are different from oneanother.

The device 70 also comprises a row decoder 72 which can be connected toa number of voltage source such as +3.3, +2.0 volts, or to +3.3 volts,and then through a voltage divider +2.0 volts is generated. The rowdecoder 72 receives an address signal and decodes and selects one of therow lines 90, 92 or 94 and supplies the +3.3 or +2.0 volts to that rowline. The device 70 also comprises a column decoder 74. The columndecoder 74 is connected to the column lines 62, 64, 66 and 68. Thecolumn decoder 74 is also connected to V_(DD) which is at +3.3 volts andV_(SS) which is at 0 volts. The column address decoder 74 also receivesaddress signals which when decoded selects a pair of column addresslines, such as 62/64 or 64/66 or 66/68. The pair of column address linesselected must be of adjacent column address lines.

The device 70 also comprises a sensing circuit 76. The sensing circuit76 measures the amount of current flow between the first and secondregions 32 and 34 of a selected ROM cell. That current flow is thencompared to the current flow measured detected from a reference cell 78and is compared by a comparator 80. The result of the comparator 80 isstored in a storage 82. Further, the device 70 comprises a switch 84 forswitching the pair of selected columns in the column decoder and forswitching the storage locations in the storage 82.

In the operation of the device 70, when an address signal is supplied tothe row decoder 72, a particular row address line, such as row addresslines 90, 92 or 94 is selected. The voltage of +3.3 (or a differentamount) is then supplied by the row address decoder 72 to the selectedrow address line, such as line 90. The column address decoder 74receives the address signal and decodes them and selects a pair ofadjacent column lines. For example, if the column address decoder 74determines that the pair of column lines 62/64 are selected, then thecolumn address decoder 72 applies, for example, the voltage +3.3 voltsto the column address line 62 and the voltage of 0 volts to the columnaddress line 64. The sensing circuit 76 measures the amount of currentflowing through the selected ROM cell 95 between the column 62 andcolumn 64. The sensing circuit 76 measures the current flow on thecolumn line 62. The amount of current flow measured is then compared tothe amount of current flow measured flowing through a reference cell 78.This comparison is performed by a comparator 80 and the result of thecomparison, as previously discussed, is a pair of possible states whichis then stored in the storage 82. Thereafter, the switch 84 reverses thevoltages applied to the pair of selected column lines 62/64. The voltageapplied to the column line 62 would then be 0 volts, while column line64 would receive the voltage of +3.3 volts. The current sensed flowingalong the column line 64 is then measured by the sensing circuit 76.This measurement of the second current flow is compared again to thecurrent flow through the reference cell 78 by the comparator 80. Theresult is that the comparator 80 selects one of the states that isstored in the storage 82. This then forms the output of the reading ofthe selected ROM cell 95. Alternative schemes in which any voltage orcurrent property that is sensitive to the threshold voltage at theportion adjacent to the region along the lower-voltage column can beconstructed by those familiar with the art of circuit design.

Referring to FIG. 7A there is shown a perspective view of a first stepof a method to make the ROM array 60 of the device 70. In the firststep, spaced apart strips of silicon dioxide 100 are formed on a planarsurface of the semiconductor substrate 20, which is of P conductivitytype. The strips 100 of silicon dioxide are formed in a directionsubstantially parallel to the direction in which the column lines 62,64, 66 and 68 are eventually formed. The spaced apart oxide strips 100can be formed by the well-known masking step in which portions of anoxide layer are removed. The portions 102 which are the spaced apartregions between adjacent oxide layers 100 are removed byphotolithography etching processes. The strips 100 of silicon dioxideare of approximately 1000 angstroms in thickness. The distance 102 bywhich adjacent strips 100 are spaced apart from one another determinesthe dimension of the first region 32 or second region 34.

In the next step, shown in FIG. 7B, N+ species are implanted into thesubstrate 20 to form the column line 62/64/66/68. Since the implant ischosen so that its energy cannot penetrate the oxide strips 100, theimplant is made in only those regions where the silicon substrate 20 isexposed. In the event the substrate is of a P conductivity type, theimplant would be of the N species type. Prior to the N+ implant, thesilicon substrate 20 may be optionally recessed to increase the L(eff).This optional step is to perform a silicon etch which is selective tothe oxide strips. This will place the columns lines 62/64/66/68 within atrench thereby extending the surface distance between them.

Referring to FIG. 7C there is shown the next step in the method ofmaking the array 60. Silicon nitride 104 is deposited on the column line64/66/68 etc. This can be done, for example, by depositing siliconnitride 104 everywhere and then using CMP polishing to planarize thestructure to stop with the surface of the silicon dioxide 100. Anotherlayer of silicon nitride 106 is then added to the structure shown inFIG. 7C. The result is the structure shown in FIG. 7D.

Photoresist 108 is then applied in the row direction of the structureshown in FIG. 7D. Photoresist in stripes 108 are deposited in spacedapart locations from one another. The photoresist 108 is patterned toopen areas where the active ROM cells are to be made. The result isshown in FIG. 7E.

Using the photoresist 108 as a mask, the portion of the silicon nitride106 that is exposed, i.e., between regions of photoresist 108, and thesilicon nitride 104 that covers the column lines 62/64/66/68 areremoved. This removal can be done by anisotropic etching of siliconnitride 106 and 104 between the photoresist strips 108. The resultantstructure is shown in FIG. 7F.

The photoresist strips 108 are then removed. The resultant structure isshown in FIG. 7G. A mask 110 is then placed over the structure. The mask110 is the same mask that is used to make the MOS transistors in otherparts of the device 70, such as the sense circuit 76, column or rowaddress decoders 74 and 72 respectively, the reference cell 78, etc. toform either the halos or the extension in the MOS transistors in otherparts of the device 70. The mask 110 is placed over selected areas suchthat the implants that follows to form the MOS transistors would alsoform the appropriate state of the ROM cell to one of a plurality of Npossible states. As shown in FIG. 7H, the mask is placed over the entireoxide region 100 of the ROM cell that is between column lines 68/66.Thus, that ROM cell would receive a state in which the first and thirdportions of the channel immediately adjacent to the first and secondregions are of the same conductivity and concentration as that of thesubstrate 20. Also shown in FIG. 7H is the ROM cell defined by theregion between the column lines 66/64. The oxide layer 100 is shown aspartially exposed (exposed on the left hand side). In thisconfiguration, the ROM cell defined by the oxide layer 100 and thecolumn lines 66/64 would have the portion of the channel immediatelyadjacent to the column line 66 be implanted with a species. In thisexample, halo implant is desired and accordingly, the species that is ofthe same type as the substrate 20 (namely P type) is then implanted intothe exposed area of the mask 110. This would result in P+ species beingimplanted through the column 66 and into the first portion of thechannel 36. The right portion of the ROM cell defined by the oxide layer100 and the column lines 66/64 would remain covered and not be subjectto the implant. Thus, the portion of the channel 36 immediately adjacentto the column line 64 would remain of the same type of conductivity andconcentration as the substrate 20.

After the implant step, the mask 110 is removed. In addition, the oxide100 which is in the exposed region between the spaced apart strips ofsilicon nitride 106 is also removed. The resultant structure is shown inFIG. 7I. Of course, the implant step described and shown in FIG. 7H mayalso be done after the oxide 100 has been removed from the exposedportion between the spaced apart strips of silicon nitride 106. The areawhere the implant has caused the change in the conductivity and/or theconcentration of the species in the substrate 20 is designated as area112, and is shown in FIG. 7I.

A photoresist mask (not shown) is placed over the structure shown inFIG. 7I. The mask would cover all the source/drain lines 64/66/68, andall the portions of the cells which is not desired to implant to changethe threshold voltage of the second portion of the channel. To decreasethe threshold voltage for the cells 50 or 150 desired, n dopant speciesis implanted into at least the second portion of the channel for theselected cells. To increase the threshold voltage for the selected cells50 or 150, p dopant species is implanted into at least the secondportion of the channel. Of course, the dopant (n or p) can be implantedinto the entire channel region of the selected cells 50 or 150. Thismasking and implant step is the same mask and implanting step that isused to set the threshold voltage for the MOS transistors in other partsof the device 70.

Thereafter, silicon dioxide 114 forming the gate oxide of the ROM cellis then deposited or formed in the exposed portion of the spaced apartsilicon nitride strips 106. After the strips of gate oxide 114 areformed, polysilicon 116 is then deposited all over the structure. Thepolysilicon 114 is then subject to a CMP polishing step with the siliconnitride strips 106 as the etch stop. The resultant structure is shown inFIG. 7J. Each strip 116 of polysilicon as will be appreciated forms thegate of the ROM cells and the polysilicon 116 connect all the gates inthe row direction. Thereafter, the silicon nitride 106 strips, which arebetween adjacent strips of polysilicon 116 are then removed leaving theresultant structure shown in FIG. 7K. A plan view of the array 60 of ROMcells is shown in FIG. 7L with the position of the extension or haloregions shown as “storage nodes.”

It should be noted that the implant step shown and described in FIG. 7Hmay be accomplished one of two methods. Each column side of eachcrossing between the gate 116 and columns 62/64/66/68 is a potentialprogramming point or “bit” (i.e. either the first or third portion ofthe channel 36 in FIG. 4 a). An opening in the resist above one of thesepoints allows the implant to program the bit. In the first method, theresist opens each side of a device selectively over each bit to beprogrammed. This requires holes whose dimension parallel to thepolysilicon strips 116 is half of the column pitch. Thus, for example,as shown in FIG. 7H, the photoresist covers the region labeled “A”, butis unmasked in the region labeled “B”. The implant is done at adirection normal to the plane of the surface of the semiconductorsubstrate 20. Therefore, region “B” will be implanted. In the secondmethod, the implant occurs at an angle other than being normal to theplane of the substrate 20. As a result, if the resist opens both sidesof a device and with an angle implant, only one device gets implanted ata time. Although two programming points are exposed, one side isshadowed by the angle of the implant and is therefore not programmed.For example, if the resist covered the oxide 100 between columns 66/64and implant occurs at an angle from “right” to “left”, because region“B” is shielded by the resist above the oxide 100, it would not beimplanted. However, region “C” would be implanted. The implant andmasking step must be done twice, once with the implant angled toward oneside or the other. The advantage is that the lithography requirement isfor holes whose dimension parallel to the polysilicon strips 116 isequal to the column pitch. From the foregoing, it can be seen that anarray 60 of the ROM cells 50 or 150 will not have any contact regionswithin the array. Thus, the array 60 can be made very compact and dense.In addition, with each ROM cell being of multi-bit, the density of thearray 60 can be further increased.

1. A method of making an array of Read Only Memory (ROM) cells arranged in a plurality of columns and rows in a semiconductor substrate of a first conductivity type having a first concentration, wherein said array of ROM cells having a plurality of ROM cells in each column and a plurality of ROM cells in each row, wherein ROM cells in adjacent columns share a common column line with each ROM cell defined by a first column line, a second column line with a channel therebetween, said channel having three portions: a first portion adjacent said first column line, a third portion adjacent said second column line and a second portion between said first portion and said third portion, and a gate for controlling the conduction of charges in said channel; said method comprising: implanting said substrate to form a plurality of spaced apart first regions of a second conductivity type, parallel to one another, in said column direction, in said substrate, wherein each first region being said common column line between adjacent columns of ROM cells; masking said array: 1) to permit implanting a first select of said ROM cells wherein for each ROM cell, implant would occur in an extension region adjacent to its associated first column line by a conductivity type or a concentration different from said first conductivity type and said first concentration to form a first state of said ROM cell; or 2) to permit implanting a second select of said ROM cells wherein for each ROM cell, implant would occur in an extension region adjacent to its associated second column line by a conductivity type or a concentration different from said first conductivity type and said first concentration to form a second state of said ROM cell; or 3) to permit implanting a third select of said ROM cells wherein for each ROM cell, implanting would occur in an extension region adjacent to its associated first column line by a conductivity type or a concentration different from said first conductivity type and said first concentration, and in an extension region adjacent to its associated second column line by a conductivity type or a concentration different from said first conductivity type and said first concentration, to form a third state of said ROM cell; or 4) to permit no implanting a fourth select of said ROM cells, wherein for each ROM cell, no implanting would occur either in an extension region adjacent to its associated first column line or in an extension region adjacent to its associated second column line, to form a fourth state of said ROM cell; and implanting said array masked to form said first, second, third or fourth state of said ROM cells; and masking said array to permit implanting a fifth select of said ROM cells wherein for each ROM cell, implanting would occur in its associated second portion of said channel for setting the threshold voltage of said cell to one of a plurality of possible voltages; and implanting said array to set the threshold voltages for said fifth select of said ROM cells. 